This invention relates to flat motors and, more particularly, to flat motors suited for use with miniaturized cassette tape recorders.
In the recent cassette tape recorder, the size reduction of the motor used has been called for in order to reduce the size of the system, and flat motors have been extensively used to meet this demand.
The usual flat motor has a construction as schematically shown in FIG. 1, having a rotor 14 rotatably supported in a housing by a shaft 12. The rotor 14 is provided with coils 18 each wound on a bobbin 16, and a magnet 20 is provided in the housing 10 to face the coils 18.
With such a construction, for enhancing the freedom of control it has been thought to minimize the gap A formed between the rotor 14 and magnet 20 or between the rotor 14 and housing 10 so as to make effective use of the magnetic field set up by the magnetic circuit. To this end, a construction where an end of the winding of each coil 18 is not found in the gap A is adopted. While the diameter of a copper wire constituting the coil 18 is around 0.2 mm, the overall winding diameter inclusive of the thicknesses of the insulation layer and self-fusion layer covering the wire, is about 0.3 mm. Therefore, in case if the end of the winding of the coil 18 is found in the aforementioned gap A, the dimension thereof has to be greater than 0.3 mm which is the diameter of the winding. With such the gap the magnetic flux density is too low to be able to permit effective use of the magnetic field, and the control property is also inferior.
Accordingly, the position of the end of the winding of the coil 18 is set to be on the inner side of the inner periphery of the magnet 20. With this construction, the end of the winding of the coil 18 is not found in the aforementioned gap A, so that it is possible to reduce the gap A to a mechanically permissible value.
However, it is intended to reduce the outer diameter of the housing 10 for reducing the size of the motor without sacrifice in the performance as shown in FIG. 2, it dictates setting the position of the winding end to be close to the shaft 12 without altering the size of the magnet 20. In this case, it is inevitable from the aforementioned positional relation between the magnet 20 and coil 18 that the radial coil size B is reduced by an amount corresponding to the extent of change of the winding end setting position. Therefore, the number of coil turns is reduced to reduce the torque, so that the performance comparable to that of the conventional motor cannot be obtained.